Union Pacific M of W Paint Schemes

[cought]

I'm modelling the UP during the late 1940's on the Idaho Division. I recall seeing silver Maintenance of Way equipment when I rode the streamliners as a little kid in the late 1950's. I recall a period in the 70's when UP started painting the MW equipment green but then went back to silver. My UP painting guide from the 1920's says "metallic" (similar to boxcar red) was the color for MW equipt.

Does anybody know when these color changes occurred?
Thanks.